A kid and his father are building a fort out of hexagonal blocks made out of pastel foam. “Now you chase me, daddy.”

Young men in their twenties are rolling down a climbing space modelled on the playground on the modernist Churchill Gardens estate, in Pimlico, east London.

One of them misjudges the descent and plunges backwards into some railings, which are mercifully padded. The only damage is to his dignity.

All this is rather bizarre behaviour in the staid surrounds of the Royal Institute of British Architects, situated in the grand Portland Place in central London. But as we’re playing on the Brutalist Playground, an installation designed to make us think about public space, freedom and imagination, it makes perfect sense.

Part of the London Festival of Architecture the playground, which ends its residency this week, is a project by the youthful architecture collective Assemble and artist Simon Terrill. It is designed to explore post-war design for play, and harks back to a time when serious architects were able to have a little fun by designing parks around or within their post-war estates, built at a time of optimism and public  investment.

In the words of Terrill: “The original playgrounds evoke a disappearing world of concrete mazes and windswept walkways. Like a Brechtian stage set rendered in concrete, they speak to a time before soft play and highly regulated public space.”

Looking at them from today’s perspective, they might seem almost wilfully dangerous. But to a generation of kids who had grown up playing on bomb sites, they must have been joyful, if inevitably resulting in a thousand grazed knees, not to mention the odd cracked head or broken tooth.

One of the ironies of the exhibition is how it had to be remade in materials acceptable to modern liability standards. And still we were told to use the installation at our own risk.

It’s interesting to think of the state of these spaces today.

Part of the playground is lovingly modelled on the slide and tower complex outside Erno Goldfinger’s Balfron Tower, a monument to the welfare state now being sold off as luxury apartments for the bankers in nearby Canary Wharf.

Clearly Goldfinger had fun. The playground compliments the tower but also in many ways goes beyond it — it’s angular, surreal, imposing and has a concrete battlement that would have afforded bragging rights to any kid holding court at the top.

In reality today, the playground is barely recognisable as such, the decades of neglect and underinvestment leading to a sorry sight barely recognisable as a former play area. That in Churchill Gardens, itself being controversially regenerated, is long gone.

I came out of the exhibition considering what we’d lost.

First up: yes, some of these spaces were pretty dangerous, compared to the soft floored, padded feel to park play areas today. I’m not advocating a return to a time when a kid had a risk of severe injury in his daily exploration of limits and exploration.

But I do worry that those explorations themselves are in danger of disappearing, which grave and depressing implications for the state of our society.

Will our roads given over to cars instead of play and decades of media hysteria over the danger of strangers lead to parents understandably not wanting to let their kids out of sight — childhood isn’t what it was.

Risk is something that should be thought about rather than avoided entirely.

We have a desperate need for more public, not private space, in the form of playgrounds, squares, fake pirate castles and complexes of slides and climbing frames.

These plans were wild, and in some cases dangerous. But they were big ideas, from people thinking without cynicism about how to make a better tomorrow.

Before leaving, I pick up some hexagons and throw them down the slanted roundabout. They bounce satisfyingly down the slope and into the shins of my companion.

I get a telling off from the gallery attendant.